Lecture 1. Getting Started
1.1 Introduction
* Objectives
* Requirements & Grading Policy
* Other information
1.2 Basic Circuit Concepts
* Electrical quantities
 current, voltage & power, sign conventions
* Circuit elements
 Passive, active and sources
* Basic laws
 Ohm’s law and Kirchhoff’s laws
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EEE 202: Circuits 1, Spring 2008
Prerequisite EEE 101
Pre- or co-requisites: MAT 274 or MAT 275, PHY 131, 132.
Instructor: Dr. NJ Tao (njtao@asu.edu)
Where: Schwada Classroom & Office 150
When: Tu and Th 3:15-4:30 pm
Office Hours: Tu and Th 2:00 - 3:00 p.m. or by appointment.
Office Location: GWC618
Class Website:
http://www.public.asu.edu/~ntao1/Teaching/ECE202/EEE202web.htm
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1.2. Basic Circuit Concepts
* Electrical quantities
 current, voltage & power, sign conventions
* Circuit elements
 Passive, active and sources
* Basic laws
 Ohm’s law and Kirchhoff’s laws
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Electrical Quantities
• Basic quantities:
– Current (I): time rate of change of electric charge
I = dq/dt
Unit: 1 Amp = 1 Coulomb/sec
– Voltage (V): electromotive force or potential
Unit: 1 Volt = 1 Joule/Coulomb = 1 N·m/coulomb
– Power (P): rate at which work is done
P=IV
1 Watt = 1 Volt·Amp = 1 Joule/sec
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Water Analogy
Base
quantity
Flow
variable
Potential
variable
Electrical
Hydraulic
Charge (q)
Mass (m)
Current (I)
Fluid flow (G)
Voltage (V)
Pressure (p)
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Current, I
• The sign of the current indicates the direction of flow
• Current due to positive & negative charge carried; the
moving direction of positive charge is conventionally
defined as direct of current.
What are charge carries in copper wire, Silicon and salt solution?
• DC & AC currents:
– direct current (dc):
batteries and some special generators
I(t)
– alternating current (ac):
household current which varies
with time
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Voltage, V
• Voltage is the difference in electrical potentials between,
e.g., two points in a circuit; it is the energy required to move
an unit charge from one point to the other.
• Voltage with respect to a common point or “ground”.
• Positive (high) and negative (low) voltages.
Circuit Element(s)
+
V(t)
–
What is electrical potential?
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Default Sign Convention
• Passive sign convention : current should enter the
positive voltage terminal
I
+
Circuit Element
–
• Passive sign convention: P = I V
– Positive (+) Power: element absorbs power
– Negative (-) Power: element supplies power
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Active vs. Passive Elements
• Active elements can generate energy
– Voltage and current sources
– Batteries
• Passive elements cannot generate energy
– Resistors
– Capacitors and Inductors (but CAN store energy)
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Independent Sources
An independent source (voltage or current) may be DC
(constant) or time-varying (AC), but does not depend on
other voltages or currents in the circuit
+
–
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Resistors
• A resistor is a circuit element that dissipates electrical
energy (usually as heat)
• Real-world devices that are modeled by resistors:
incandescent light bulbs, heating elements (stoves,
heaters, etc.), long wires
• Resistance is measured in Ohms (Ω)
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Ohm’s Law
v(t) = i(t) R - or p(t) = i2(t) R = v2(t)/R
V=IR
[+ (absorbing)]
i(t)
The
Rest
of the
Circuit
+
R
v(t)
–
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Open Circuit
• What if R=?
i(t)=0
The
Rest of
the
Circuit
+
v(t)
–
• i(t) = v(t)/R = 0
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Short Circuit
• What if R=0?
i(t)
The
Rest of
the
Circuit
+
v(t)=0
–
• v(t) = R i(t) = 0
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Resistors in Series
Two or more elements are in series if the current that flows
through one must also flow through the other.
R1
R2
In series
I1 = I 2
Not in series
R1
R2
I 1 ≠ I2
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Resistors in Parallel
• Two or more elements are in parallel if they are connected
between (share) the same two (distinct) end nodes;
• The voltages across these elements are the same.
R1
R1
R2
R2
Parallel
Not Parallel
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Kirchhoff’s Laws
• Kirchhoff’s Current Law (KCL)
– sum of all currents entering a node is zero
– sum of currents entering node is equal to sum of currents leaving
node
– Conservation of charge
• Kirchhoff’s Voltage Law (KVL)
– sum of voltages around any loop in a circuit is zero
– Conservation of energy
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KCL (Kirchhoff’s Current Law)
i1(t)
i5(t)
i2(t)
i4(t)
i3(t)
The sum of currents entering the node is zero:
n
 i (t )  0
j 1
j
Analogy: mass flow at pipe junction
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Class Examples
• Drill Problems 1, 2, 4
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